Lets say just enough space on one side to fit the sub, so 16.5 x 16.5 is what we will shoot for for the first two dimensions. So, multiply 2.3 (assuming the displacement is .2 for the driver) by 1728, then divide that by 15(subtracted 1.5" for wall thickness), then divide it by 15 again, and that answer will be the final internal dimension. Then, you add 1.5 to each dimension because of wall thickness. You end up with 16.5 x 16.5 x 19.164. Thats not optimal dimensions, just an example.
Man, I hope that makes sense to you.